1. Technical Field
The present invention relates to a method for manufacturing a flexible printed circuit board, and especially to a method for manufacturing a multilayer flexible printed circuit board having different number of layers in different areas.
2. Discussion of Related Art
Flexible printed circuit boards (FPCBs) have been widely used in electronic products such as mobile phones, printing heads and hard disks. In these electronic products, some parts may move relative to a main body. FPCBs can assure power supply and signal transmission in such environment due to their excellent flexibility.
FIG. 30 shows a multilayer FPCB structure, which has different numbers of layers in different areas; in other words, there are thick areas and thin areas within the same FPCB. The thick area can have a higher circuit density whilst the thin area exhibits higher flexibility.
FIGS. 25-30 show a process for manufacturing such a type of FPCB. As is shown in FIGS. 25 and 26, a first copper clad laminate (CCL) 41, a binder layer 45 and a second CCL 42 are laminated. As is shown in FIG. 27, dry films 412, 422 are applied on the first CCL 41 and the second CCL 42 respectively and then the dry films 412, 422 are exposed and developed. Because there is a cliff-like thickness difference between the first CCL 41 and the second CCL 42, a gap 46 is formed at ‘cliff’.
As is shown in FIG. 28, when the first CCL 41 and the second CCL 42 are etched using an etchant and the dry films 412, 422 are removed, the etchant can seep into the gap 46 and react with dielectric layers in the first CCL 41 and the second CCL 42. As a result the dielectric layers may peel from the CCL.
Referring to FIG. 29, a third CCL 43 and a fourth CCL 44 are laminated with the first CCL 41 and the second CCL 42. Referring to FIG. 29, a via hole 47 is formed. The via hole 47 can be made by drilling or by laser ablation. After the via hole 47 is formed a conductive layer is formed on a surface of the via hole 47 by electroless plating or electroplating. In the plating process the dielectric layer of the second CCL 42 is exposed to a plating solution, thereby forming a number of copper lumps thereon. These copper lumps can pierce dry film that is applied onto the second CCL 42 in the next pattern-forming process, and etchant used for developing the dry film can react with dielectric layer or copper layer of second CCL 42 and cause poor quality products to be formed.
In the aforementioned process for manufacturing multilayer FPCB that has different number of layers in different areas, a cliff-like structure between different CCLs can causes a series of quality problems, therefore there is a desire to develop a process that can solve aforementioned quality problems.